A mini-duct system from Unico offered a solution for air conditioning 1950s ranch-style Eichler houses in California. Main trunks on the roof feed the smaller ducts that are snaked in the wall stud bays.

It's tough to stay within budget when you're trying to add air conditioning to an old house. Let's say your heating's fine—you have radiators or a radiant floor system; however, to condition a house built before central air, you'll have to find a home for the bulky compressor or air handler. Do you shove the trunks aside in the attic? Take up basement space? Ducts to deliver cool air to all the rooms can chop up the building's historic fabric. Are you willing to sacrifice closet space? Here we'll survey the options, and then we'll show you two historic buildings whose stewards came up with creative ways to tuck in air conditioning.

Let's start with the two essential yardsticks of an air conditioning purchase: capacity and efficiency. Cooling capacity should be determined by a contractor and be based on a load analysis using the Air Conditioning Contractors of America Manual J. The contractor might also use a computer program that accounts for the building's particular construction, such as surface areas of exterior walls, insulation levels, window glazing, and air infiltration. At best, old houses have some attic insulation, but the walls are rarely insulated. By today's standards, the windows are loose. That's why there's no off-the-shelf answer or square-foot solution to home air conditioning. Cooling capacity is measured in terms of BTUs or tons of cooling (one ton equals 12,000 BTUs). Once you know how much cooling your home needs, you can begin to shop.

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As you compare air conditioners, consider each unit's efficiency rating or Seasonal Energy Efficiency Ratio (SEER), then calculate the payback—that is, the number of years before your investment breaks even. Any air conditioner with a SEER more than 12 will provide reasonable utility bills; however, our vintage buildings with their loose windows, doors, and gaps around baseboards and moldings, tend to have high levels of infiltration. In humid climates, infiltration brings in significant levels of moisture. The goal of installing central air is to lower the ambient temperature, but even more, to control humidity. Lower humidity may allow you to be comfortable at a higher thermostat setting, which means the compressor won't run as often.

For this reason, premium air-conditioning systems may be the best choice for old houses. Most premium air-conditioning units (15 SEER or above) have two-stage compressors or dual compressors and adjustable speed blowers. This gives you the best humidity control. You can go to a much higher SEER air conditioner and have all the bells and whistles. (Trane's latest model rates 19.5 SEER; Carrier's is a close second.) The capital cost will be higher, but the investment could make sense in climates with long cooling seasons and high electrical rates.

"In Philadelphia, St. Louis, Washington, D.C., or the Ohio River Valley," says Jim Crawford, director of Regulatory Affairs at Trane, "you'd love to have air conditioning two weeks a year, and you could enjoy it three to five months. But in Florida or Texas, you need it nine or ten months a year." Air conditioners either come with an energy guide label, or your dealer will be able to provide you with the information. The label allows you to compare the equipment to similar systems on the market, and it shows you how to check the cooling days per year before calculating the payback on your capital investment. The payback will be different if you only turn on your air conditioning two weeks a year, compared to nine months. As with any major investment, you should not only consider your comfort, but also the value your investment would bring if spent elsewhere, such as on a kitchen or bath upgrade.

You can install conventional ducts for a central-air system in homes built without them if you're willing to sacrifice closet space and box in corners of rooms. The basic installation could cost less than more specialized systems, but the final project may run more by the time you're finished replastering and redecorating. When there's no space to spare, your alternative might be a system that reduces or eliminates the ducting.

Marek Mroz of American Vintage Home, Inc., in Wilmette, Illinois, prefers to use 3" mini-ducts (manufactured by Unico or SpacePak). Mini-duct systems send high-velocity air through small, round ducts that can be snaked through walls or between floor joists. Although a mini-duct system is unobtrusive when installed, the mini-ducts are also much smaller in cross section than standard forced-air ducts; and because of their size, they can only put out sufficient air to adequately cool a room by spitting it out fast. Therefore, you need to tuck them into the corners of a room and not above chairs where you'll be sitting. In Chicago's suburbs, where it's not only freezing in winter but humid in summer, a typical old house requires four to five tons of cooling (five tons is the maximum a single system can cool). For three-story homes, Mroz says he'll install two mini-duct systems that cool the upper two floors with a four-ton system and the bottom two floors with a two-ton. "This gives you the option of running the upstairs alone," he says. Of course, this affects cost. A single system costs $12,000 to $16,000, but two systems still doesn't quite double the cost: $15,000 for the upper and $12,000 for the lower.

If you have a solid masonry building (all brick or concrete block) and no attic or closet space to house the mini-duct's air handler, you could also think about a ductless system. Mroz says these are also a good choice for a third-floor attic, which is typically finished as a playroom or office. "The third floor heats up quicker, and the thermostat on a lower floor may not even sense that there's a need for cooling," he says. With the noise of the compressor outside the house and wall- or ceiling-mounted "evaporator" unit inside, the ductless air-conditioning units are quieter than the average window air conditioner, thanks to manufacturers (Mitsubishi, Sanyo, and others) that pay close attention to fan noise. The evaporators require a conduit-sized line installed on the outside of the house, but that can be tucked into a jog or run up the back side of the building.

The main problem, according to Mroz, is the evaporator unit's decidedly office-like appearance inside. The evaporator's modern housing can't be covered up without blocking air flow. Also, the equipment isn't cheap. Two tons of cooling—enough for one large room—costs approximately $5,000. Are you thinking wistfully about your much-maligned (and cheap) window air conditioner?

You can eliminate the visual blight that comes from window air conditioners and provide whole-house cooling with through-the-wall air conditioners. This is the cheapest option. Models that provide up to 12,000 BTUs of cooling run on 110V service and cost about $500, plus installation. You can find units that run on 220V that provide 22,000 BTUs of cooling. These cost $700 to $900. (Compare that price to the two-ton ductless air-conditioning unit.) Models with higher energy efficiency ratings also have quieter fans and remote controls. According to Mroz, Carrier and Trane are making units that are pretty quiet. The only drawback is that your room may look like a motel, and you'll have the yearly maintenance of covering up the exterior grille. At least the grille would be flush with the façade, and not projecting like a window unit. Then again, you're cutting holes in your house.

If you still can't decide between a mini-duct system and a ductless alternative, picture how the product will look and sound inside and outside the house. Make sure you compare expenses, both capital costs and operating costs. In the interests of preservation, it's easy to buy more air conditioning than you need, but it's also easy to feel some buyer's remorse when you're staring at the modern face of an appliance in your period-decorated room and thinking the two, somehow, don't go together.

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